A device of the kind defined above is described in U.S. Pat. No. 4,433,770 and the corresponding French published patent application FR No. 2 495 255A, in which the intervention member is mounted for rotation and is adapted to come into engagement on the outer periphery of the hub. The latter is part of one of the said coaxial parts of the device, and includes a fixed abutment means for engagement by the intervention member.
A return spring is attached to the latter. These arrangements enable resilient members of lower stiffness, forming part of the torsion damping device, to be restrained from acting by themselves, so that these resilient members can be inoperative when not required.
The intervention member enables shocks that occur when the speed of rotation is lower than the slow running speed of the engine, for example when declutching followed by clutching causes stalling of the engine, to be eliminated. However, these arrangements call for a large number of components, mainly because of the rotatable mounting of the intervention member. They are also bulky.